revert: restore conversation unread count feature flag (#14623)

This reverts #14610 so conversation unread counts are again controlled
by the `conversation_unread_counts` feature flag across the API,
ActionCable broadcasts, notifier/listener paths, and dashboard sidebar
fetching.

## Closes
- None

## What changed
- Restores feature-flag checks for conversation unread count reads and
broadcasts.
- Restores the dashboard feature flag constant and sidebar/store
behavior for disabled unread counts.
- Restores the specs that cover disabled-feature behavior.

## How to test
- In an account with `conversation_unread_counts` enabled, verify
sidebar unread counts are fetched and updated in real time.
- Disable `conversation_unread_counts` for the account and verify unread
count requests/broadcasts are skipped.

app/javascript/dashboard/store/modules/conversationUnreadCounts.js

@@ -48,6 +48,9 @@ export const actions = {
       // Ignore errors so the sidebar can continue rendering without badges.
     }
   },
+  clear({ commit }) {
+    commit(types.SET_CONVERSATION_UNREAD_COUNTS, {});
+  },
 };
 
 export const mutations = {

app/javascript/dashboard/store/modules/specs/conversationUnreadCounts/actions.spec.js

@@ -39,4 +39,15 @@ describe('#actions', () => {
       expect(commit).not.toHaveBeenCalled();
     });
   });
+
+  describe('#clear', () => {
+    it('clears unread counts', () => {
+      actions.clear({ commit });
+
+      expect(commit).toHaveBeenCalledWith(
+        types.SET_CONVERSATION_UNREAD_COUNTS,
+        {}
+      );
+    });
+  });
 });